The present invention relates to the timing of the instant of introduction of fuel into the combustion chamber of a spark ignited internal combustion engine in which the charge is stratified.
The concept of a stratified charge is known from many engine applications as will be described below. The basis of operation is that the overall mixture is too lean to burn correctly but by ensuring an uneven distribution of the fuel in the combustion chamber immediately before the spark, it is possible provide a richer mixture in the immediate vicinity of the spark plug electrodes to guarantee proper ignition, the powerful flame then being able to propagate and ignite the remaining charge which becomes progressively leaner.
One application in which a stratified charge is required is when the quantity of fuel is being used instead of the intake air volume to regulate the engine power output. Here, when operating under part load, there will be excess air and consequently it is essential in view of the overall weakness of the mixture to resort to a stratified charge if proper ignition is to be achieved. This is just one example of a lean burn and others will also come to mind.
Charge stratification also occurs unintentionally when fuel is injected either directly into the cylinder or into the intake port while the inlet valve is open. The fuel injection creates a local concentration of the fuel in the combustion chamber and its timing has been found to be critical. Good performance is achieved if the rich mixture lies within the vicinity of the spark plug electrodes at ignition but not otherwise. Here the charge is meant to be homogeneous but the importance of the injection timing suggests otherwise.
In the case of injection into the intake port, the best performance is produced if injection occurs in a narrow time window while the inlet valve is open but the timing is so critical and the performance is so degraded outside this window that it is generally preferred to time the injection to take place while the intake valve is closed. Here, the efficiency is not optimised but the injection timing is not critical. This ensures a homogeneous mixture but the efficiency is maximised if a weak mixture is burnt while the charge is correctly stratified.
A still further application in which charge stratification is met is in the case of two-stroke engines. A two stroke engine running under part load conditions must resort to charge stratification because the mass of the trapped gas can only be controlled within narrow limits and the regulation of the power is carried out by limiting the fuel supply.
It has been proposed in the case of a two stroke to provide in addition to the transfer passage and the exhaust port a valve controlled fuelling port in which fuel is accumulated during the engine cycle, the fuel being introduced into the combustion chamber at a time when it will not find its way to the exhaust port. As in the case of direct injection into the cylinder of a four stroke engine, the timing of such introduction of the fuel into the combustion chamber is important if advantage is to be taken of the charge stratification to maximise the engine efficiency.